1. Technical Field
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an In-Plane Switching (IPS) mode LCD in which a color filter layer and a thin film transistor are formed on the same substrate to drive the liquid crystal using a fringe field.
2. Description of the Related Art
In general, a cathode ray tube (CRT) has been most widely used among display devices for displaying image information on a screen. However, CRTs are large and heavy compared with the display area. With the development of the electronic industry, the use of display devices has expanded to personal computers, notebook computers, wireless terminals, vehicle instrument panels, electronic display boards and the like. Also, due to the development of information communication technology, since it is possible to transmit a large amount of image information, increasing importance has been placed on a next generation display device capable of processing and displaying the large amount of image information.
Such next generation display devices are required to realize lighter, thinner, shorter and smaller characteristics, a high luminance, a large-sized screen, low power consumption and a low price. Among such next generation display devices, liquid crystal display devices (LCD) are used as they have excellent resolution compared with other flat displays and a fast response time considerable to that of the CRT in implementing a moving picture.
More specifically, twisted nematic (TN) mode LCDs have been generally used. In the TN mode type LCD, after electrodes are formed on two substrates and liquid crystal directors are aligned twisted by 90°, a driving voltage is applied to the electrodes to drive the liquid crystal directors. However, the TN mode LCD has a narrow viewing angle.
Recently, LCDs employing a new mode are being actively researched to increase the viewing angle. In-plane switching (IPS) mode LCDs, optically compensated birefringence mode LCDs, etc. are examples of such LCDs.
The IPS mode LCD generates a horizontal electric field to drive the liquid crystal molecules in a horizontal state with respect to the substrates by forming two electrodes on an identical substrate and applying a voltage between the two electrodes. In other words, the longer axis of the liquid crystal molecule does not stand up with respect to the substrates. To this end, the IPS mode LCD has a small variation in the birefringence of liquid crystal according to a visual direction and thus has an excellent viewing angle characteristic compared with the TN mode LCD.
FIG. 1 is a sectional view of a related art IPS mode LCD. Referring to FIG. 1, a related art IPS mode LCD is formed by attaching a first substrate 118 and a second substrate 119 facing the first substrate 118, and interposing a liquid crystal layer 130 therebetween. A metal film is first deposited on the first substrate 118 and is patterned to form a plurality of gate lines and a plurality of gate electrode 109 branched from the respective gate lines and formed at a thin film transistor region.
Next, a gate insulating layer 120 is formed on an entire surface of the first substrate including the gate electrode 109, and then a semiconductor layer 115 forming an ohmic contact layer with an active layer 115a is formed on a predetermined region of the gate insulating layer 120. On the gate insulating layer 120, a data line 110 forming a matrix configuration together with the gate line is formed.
In the course of forming the data line 110, source electrode 116 and drain electrode 117 of a thin film transistor are formed along with the data line 110. Also, a common line and a common electrode 113 are formed to be parallel with the gate line 110. Alternatively, the common line and the common electrode 113 may be formed on the same layer as the gate electrode 109.
On the entire surface of the first substrate 118 constructed as above, a passivation film 128 is formed. After that, a pixel electrode 114 is formed to be electrically connected with the drain electrode 117 and be parallel to the data line 110. On the entire surface of the first substrate 118 constructed as above, a first orientation film 129 is formed.
On the other hand, on the second substrate 119, a black matrix 121 for preventing light from being leaked is formed. A color filter layer 122 consisting of color patterns of red (R), green (G), and blue (B) is formed between the black matrixes 121. On the color filter layer 122, an overcoat layer 123 for planarizing an upper surface thereof and protecting the underlying color filter layer 122 is formed. Next, a second orientation film 126 is formed on the overcoat layer 123.
Edges between the first and second substrates 118 and 119 are sealed by a seal pattern (not shown) to prevent the liquid crystal layer 130 interposed between the first and second substrates 118 and 119 from being leaked. Attaching of the first and second substrates 118 and 119 is determined by a margin on design of each substrate. Generally, preciseness of a few micrometers is required. If the attaching margin deviates from an allowed margin, light is leaked and accordingly a desired characteristic is not obtained.
To realize high definition, integration of the LCD has increased and an interval between devices has narrowed. Accordingly, if even a minor error in attachment exists, corresponding devices are formed at misaligned sites, so that color reproducibility and production yield are lowered.
However, in the related art IPS mode LCD where the color filter layer and the thin film transistor are formed on different substrates, the attachment preciseness is lowered due to alignment margin between the color filter substrate and the array substrate. Also, since the color filter substrate and the array substrate are separately formed, process time is increased, so that production yield is lowered.